Update README with secret field documentation

- Document secret parameter in offer creation examples
- Add Protected Offers section with detailed usage
- Update API reference for create() and answer() methods
- Show hasSecret flag in discovery responses

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/rondevu-client",
-  "version": "0.7.5",
+  "version": "0.7.6",
   "description": "TypeScript client for Rondevu topic-based peer discovery and signaling server",
   "type": "module",
   "main": "dist/index.js",